Part I. Photochemical and Photophysical Studies of Guanine Derivatives: Intermediates Contributing to its Photodestruction Mechanism in Aqueous Solution and the Participation of the Electron Adduct

Abstract

The low-intensity steady-state (254 nm), microsecond flash and nanosecond (266 nm) laser photolysis of some guanine (Gua) derivatives in aqueous solution were studied. A photodestruction yield between 10(-3) and 10(-2) at a base concentration of 75 microM was determined for 254 nm irradiation at room temperature using high-performance liquid chromatography. This yield decreases with increasing purine concentration. For a similar concentration of the purine bases (2 +/- 1) x 10(-5) M, the yield increases as follows: Gua approximately 9-ethylguanine < deoxyguanosine approximately guanosine (Guo) < guanosine 5'-monophosphate. At concentrations higher than 2 x 10(-4) M the Gua derivatives' photodestruction yield seems to converge to a limiting value of the order of 10(-4). This behavior is explained in terms of self-quenching and aggregation effects which deactivate the excited states of the bases. The yields of electron photoejection have been determined in the nanosecond laser photolysis (0.083) and in the low-intensity steady-state (5.8 x 10(-3)) for Guo. Competition experiments using electron scavengers suggest that the electron adducts of the bases are one of the principal species participating in the photodestruction mechanism of these monomeric Gua. Close to 75% of the total destruction yield has contributions from initial reactions of the photojected electron at neutral pH. The quantum yield of photodestruction of Guo increases when the pH is increased as follows: 4.7 x 10(-3) (pH 1.1), 6.5 x 10(-3) (pH 2.9), 7.7 x 10(-3) (pH 7.5) and 8.1 x 10(-3) (pH 11.9). This dependence on pH and the electron scavenger experiments provide further evidence for the radical anion or its protonated form as one of the principal species involved in the photodestruction of the bases at the different pH. Under oxygen saturated conditions a 22% increase in the destruction yield is observed for Guo. However, for the dinucleotides adenylyl (3'-->5')-guanosine and thymidylyl (3'-->5')2'-deoxyguanosine, the participation of the electron is 41 and 36%, respectively, suggesting that going into a more DNA or RNA-like structure, the participation of the electron adducts species in the photodamage of DNA and RNA decreases. A mechanism of photodestruction for the Gua derivatives is proposed which takes into account these findings.

Magnetic field-enhanced photoinization of 6-methylpurine

“…At this point the question was whether other purine bases, particularly adenosine (Ade) and guanosine (Gua), exhibit the same phenomenon. The hydrated electron band is indeed observed in the transient absorption spectra of Ade and Gua [13]. However, we repeated the experiments described for P but we did not observe an enhanced hydrated electron yield.…”

Part II. Mechanism of Formation of Guanine as one of the Major Products in the 254 nm Photolysis of Guanine Derivatives: Concentration and pH Effects

“…We have centered our studies on the continuous photolysis of aqueous solution of the monomeric Gua units, aiming to provide background information to explain the photochemical behavior observed for these constituents in high-order nucleic acid polymeric structures. In the first part of this series of papers (12), we reported the extent of participation of the reactive intermediates and the photodestruction yields for the 254 nm irradiation of the Gua derivatives, at room temperature, determined from changes in absorption at 252 nm or using high-performance liquid chromatography (HPLC). A possible mechanism for the photodestruction of the bases taking into account the effects of pH, self-quenching and aggregate formation was presented.…”

Deep-UV biological imaging by lanthanide ion molecular protection

“…Without Ln ion (Fig. 5(a)), nucleobases and aromatic amino acids can be ionized [12][13][14][15]18,20,21]. Reaction with surrounding molecules such as nucleobases, aromatic amino acids, and O 2 can also occur, resulting in irreversible formation of photoproducts (i.e.…”

“…Relaxation of excited nucleobases and aromatic amino acid can also occur, without causing the molecular photodegradation, through thermal and radiative decays [21,[47][48][49]. However, the remainder of the energy can thereby transfer to surrounding molecules, including O 2 , resulting in ionization [18] and reactions with surrounding molecules [12,13,15,18,20,21], including generation of ROS, which can modify molecules that would otherwise be transparent to DUV light [18].…”

“…However, the practical use of DUV light for molecular imaging is limited. DUV light can destroy or denature biological molecules due to absorption [10][11][12][13][14][15][16][17][18][19][20][21], which restricts its use in quantitative, repetitive, and/or high signal-to-noise ratio (SNR) analyses of target molecules in a limited detection volume by DUV imaging, especially for samples involving trace amounts. Although a variety of advanced DUV imaging techniques measuring absorption [22][23][24], resonance Raman scattering [25], fluorescence [26][27][28][29][30], and photoacoustic signals [31,32] of DUV-absorptive molecules have been developed recently, the destructive nature of DUV light limits some unique benefits of these techniques in exploiting biological molecules, which could be measured with high sensitivity if the photodegradation could be avoided.…”